1. Technical Field
The present invention relates to lithography, and particularly to a phase shifting mask.
2. Description of Related Art
In semiconductor fabrication, lithography is an important and indispensable technique which is used to transfer circuit layout patterns through a mask onto predefined locations on a semiconductor wafer. Many processes of semiconductor fabrication, such as etching and ion implantation, require the use of lithography. In a lithographic process, resolution and depth of focus (DOF) are two major checkpoints used to appraise the quality of the pattern definition. A high level of integration requires a high resolution of pattern definition since the feature size is very small. To increase the resolution, a laser source with a very short wavelength, such as a krypton (Kr) deep ultra-violet laser with a wavelength of 2,480 xc3x85 (angstrom), is used as the exposure light in the lithographic process. The use of a short-wavelength exposure light, however, will result in a shallow DOF. To allow high resolution and good DOF, one solution is to use the so-called phase shifting mask (PSM).
Multiphase phase shifting mask shifting an exposure light by about 90xc2x0 or 270xc2x0 in phase is phase shifting mask used to improve resolution of hole patterns with low duty ratio. However, proximity effect could be worse due to isolated/dense surrounding difference. Specifically, the assist hole features, shifting the exposure light by about 90xc2x0 or 270xc2x0 in phase and surrounding the hole feature of a multiphase phase shift mask, cause environment phase differences that deteriorate the resolution of hole patterns with low duty ratio. For the foregoing reasons, there is a need for a multiphase phase shifting mask that can prevent the isolated/dense proximity issues caused by environment phase differences.
It is an object of this invention to provide a multiphase phase shifting mask that satisfies the need identified in the background section.
Other objects and advantages of this invention will become apparent to those of ordinary skill in the art having reference to the following specification in conjunction the drawings.
The present invention is directed to a phase shifting mask. The phase shifting mask comprises a transparent substrate; a lightly transparent material shifting an exposure light by about 180xc2x0 in phase on the transparent substrate; and a plurality of first transparent hole features and second transparent hole features phase shifting the exposure light by about 0xc2x0 in phase on the transparent substrate. The phase shifting mask further comprises a plurality of assist transparent hole features alternatively located between the second transparent hole features.
The assist transparent hole features comprises a first group of transparent hole features shifting the exposure light by about 90xc2x0 in phase and a second group of transparent hole features shifting the exposure light by about 270xc2x0 in phase alternatively arranged with each other. Each of the first, the second, and the assist transparent hole features is adjacently surrounded by the lightly transparent material.
In a preferred embodiment of the present invention, the lightly transparent material, which is MoSiOxNy or MoSi, has a transmittance of about 6%.
By the lightly transparent material, the present invention can increase the image contrast to prevent the isolated/dense proximity issues caused by environment phase differences.